1. Field of the Invention
This invention relates to a method of and apparatus for effective erasing noise developing in a reproduced visible image due to repeated use of the stimulable phosphor sheet in a radiation image information recording and reproducing method where the stimulable phosphor sheet is exposed to a radiation to record a radiation image therein and then exposed to a stimulating ray to emit light in the pattern of the stored image, the emitted light is converted to an electric signal, the electric signal is processed as desired, and a visible image corresponding to the radiation image is reproduced by use of the electric signal.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. patent application Ser. No. 220,780, U.S. Pat. No. 4,387,428, Japanese Unexamined Patent Publication No. 56(1981)-11395, U.S. Pat. Nos. 4,258,264, 4,276,473 and 4,315,318, it has been proposed to use the stimulable phosphor for recording a radiation image of the human body for medical diagnosis, or the like. Specifically, the stimulable phosphor is first exposed to a radiation to have a radiation image stored therein, and is then scanned with a stimulating ray which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image of a quality suitable for viewing and diagnostic purposes.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube. The stimulable phosphor sheet used in this method may be in any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For economical reasons, it is desirable that the stimulable phosphor sheet is reusable in many separate radiographic operations.
In order to reuse the stimulable phosphor sheet, it is necessary that the stimulable phosphor sheet to be reused is made completely free from the previously stored radiation image. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should be eliminated when the sheet is scanned with a stimulating ray of a sufficient intensity to cause light to emit therefrom in the pattern of the stored radiation image in the course of the radiation image recording and reproducing process as described above. Actually, however, the stored radiation energy cannot be completely eliminated only with the stimulating ray used to scan the stimulable phosphor sheet during the aforesaid process. Thus a part of the previously stored radiation image remains in the once-used stimulable phosphor sheet and inconveniently causes noise to occur in the visible image reproduced from the stimulable phosphor sheet when it is reused. In order to successfully reuse the stimulable phosphor sheet, any such residual radiation image thereon must be erased completely before reuse.
The inventors conducted experiments to find what level of radiation energy of the residual image in the resued phosphor caused noise to develop in the reproduced visible radiation image to an extent adversely affecting diagnosis. From the results of these experiments, it has been found that, in order to eliminate the detrimental noise due to the residual image, the radiation energy of the radiation image stored in the phosphor must be erased to the order of 10.sup.-4 to 10.sup.-6. Stated differently, the original radiation energy stored in the phosphor must be erased to a level between 0.01 and 0.0001 when the maximum of the original level is 100.
Further, a stimulable phosphor contains a trace of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiations and cause the stimulable phosphor sheet to store the emitted radiation energy even when the sheet is not being used in radiography. These traces of radioactive isotopes also constitute a cause of the noise developing in the reproduced visible radiation image. Furthermore, a stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted from other X-ray sources and stores the energy thereof. These types of radiation energy (hereinafter referred to as fog) undesirably stored in the stimulable phosphor sheet also cause noise to appear in the visible radiation image reproduced from a reused stimulable phosphor sheet. The level of the fog developing in the phosphor is generally about 0.1 to 0.01 based on the maximum of the stored original radiation energy which is taken as 100 as described above. It has also been found that the fog must be erased to a level between about 0.01 and 0.0001 in order to prevent the fog from developing detrimental noise in the next radiographic operation.
In order to avoid noise occurring in the reproduced visible radiation image due to the noise originating from the radiation image previously stored in a stimulable phosphor sheet and due to the fog developing during the storage of the sheet, the Applicant has proposed in U.S. patent application Ser. No. 168,795, U.S. Pat. No. 4,400,619, to expose the stimulable phosphor sheet to light having the wavelength within range of the stimulating light wavelength of the phosphor before recording a radiation image in the stimulable phosphor sheet, thereby to discharge the detrimental radiation energy therefrom to an acceptable extent. With this method, however, erasing of the residual radiation image and fog in the stimulable phosphor sheet must be effected immediately before using the sheet for radiography. This is necessary to minimize the fog developing in the stimulable phosphor sheet after the erasing is conducted. Further, in order to erase the radiation energy of the previously stored radiation image to the order of 10.sup.-4 to 10.sup.-6, the phosphor must be exposed to a high illuminance for a long length of time, for example to 30,000 1.times. for 100 to 1,000 seconds using a tungsten-filament lamp. This necessitates a large-scale erasing apparatus and, in addition, erasing must be started a considerable length of time before a radiograph is to be taken. Thus it is very difficult in practical use to carry out such a troublesome erasing operation each time a radiograph is to be taken. Further, it is very inconvenient in practical use to install a large-scale erasing apparatus in the vicinity of the radiographic equipment.
The inventors conducted various experiments with respect to the aforesaid two kinds of causes of noise, and have found that the erasability thereof differs considerably between the residual radiation image having a level of radiation energy of about 0.1, which remains in a stimulable phosphor after the radiation energy of the previously stored image is erased from the level of 100 to about 0.1, and the fog accumulated in the stimulable phosphor to a level of about 0.1 when the phosphor is allowed to stand, even though the level of radiation energy is the same. That is to say, after the previously stored image is erased from the level of 100 to about 0.1, the resulting residual image suddenly becomes difficult to erase. For example, the previously stored image can be erased to a level of about 0.1 when exposed to light at an illuminance of 10,000 1.times. for about 10 seconds, but it takes about 100 seconds for the resulting residual image to be further erased to a level of 0.01. In contrast, the fog of the level of about 0.1 can be erased in less than 1 second at an illuminance of 10,000 1.times..
Based on the above-mentioned observations, the inventors have already proposed in U.S. patent application Ser. No. 338,734, U.S. Pat. No. 4,439,682, a noise erasing method comprising two erasing steps wherein the first erasing which requires a long period of time to erase the previously stored image is carried out at an appropriate point of time after the radiation image is stored in a stimulable phosphor and scanned with a stimulating ray, and the second erasing for the fog which can be erased quickly is conducted immediately before the next radiographic operation.
According to this proposed method, the first erasing in which the radiation image previously stored in the stimulable phosphor is to be erased at a high illuminance for a long period of time by use of a large-scale apparatus can be carried out at an appropriate point of time after the stimulable phosphor sheet has been used for a radiographic operation. After the first erasing is finished, the stimulable phosphor sheet can be transferred to the vicinity of the site where it is to be used for the next radiographic operation. Thereafter, immediately before the next radiographic operation is to be started, the second erasing can be conducted for a short length of time to erase the fog, if any, by use of a simple small-scale apparatus. Thus this method can effectively eliminate the causes of noise and provide a noise-free reproduced visible radiation image.
However, the above-mentioned method is not economical since large energy is required to conduct the first erasing. Thus, a need exists for a method and apparatus which can erase noise more economically.